Problem: Which of these numbers is prime? ${5,\ 15,\ 25,\ 27,\ 88}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 5 are 1 and 5. The factors of 15 are 1, 3, 5, and 15. The factors of 25 are 1, 5, and 25. The factors of 27 are 1, 3, 9, and 27. The factors of 88 are 1, 2, 4, 8, 11, 22, 44, and 88. Thus, 5 is a prime number.